Cubic boron nitride, hereinafter referred to as cBN, is generally employed as a material for a cutting tool, since the same is the hardest material next to diamond and has extremely stable thermal and chemical properties. Further, cBN has an excellent thermal conductivity even compared to diamond, and it is expected that it will be used as a heat sink or radiation substrate for a semiconductor laser or the like. However, it is extremely difficult to make a large-sized single crystal of cBN, and hence a sintered compact formed of cBN grains of several micrometers is considered to be a practicable material.
Since it is extremely difficult to directly sinter cBN grains or powder independently, a metal such as Al or Co, or a carbide such as TiC, or a nitride such as TiN is generally employed as a binder for sintering the cBN grains. In a cBN sintered compact containing the aforementioned binder, however, the binder defines a continuous phase among the cBN grains, and hence the hardness and thermal conductivity of the sintered compact are significantly reduced so that it is difficult to satisfactorily obtain the excellent properties originally provided by the cBN.
Japanese Patent Laying-Open No. 54-33510 or Material Research Bulletin, Vol. 7 (1972), pp. 999 to 1004 discloses a first known method of making a cBN sintered compact not containing any binder with a starting material of atmospheric pressure type BN, hexagonal boron nitride: hereinafter referred to as hBN, by directly converting hBN to cBN under superhigh pressure/temperature conditions of 6.5 GPa at least 1800.degree. C. and simultaneously sintering the same. However, such a method of making a cBN sintered compact by direct conversion requires extremely high pressure/high temperature conditions. Even if these conditions are satisfied, the known method is insufficient in its reproducibility, so that it is not suitable for industrial production.
On the other hand, each of Japanese Patent Laying-Open Nos. 58-176179 and 59-57967, Japanese Patent Publication Nos. 59-5547 and 60-28782, discloses a further known method for producing a cBN sintered compact under relatively low pressure/temperature conditions as compared with the aforementioned direct conversion method and with an excellent reproducibility. According to the further method, a small quantity of alkaline earth metal boron nitride is added to or diffused/contained in the hBN powder or in a sintered compact thereof, which is treated under a thermodynamically stable pressure condition for hBN at a temperature exceeding 1350.degree. C. The further method is adapted to convert hBN to cBN with a catalyst of the alkaline earth metal boron nitride while bonding the cBN grains with each other simultaneously with such conversion. It is said that, according to the further method, the material can be treated at a temperature exceeding 1350.degree. C., which is the eutectic temperature of the alkaline earth metal boron nitride and hBN, to obtain a strong sintered compact which is formed of only cBN grains of 3 to 10 .mu.m in unit grain diameter and substantially not containing any impurity.
U.S. Pat. No. 4,772,575 (Ota et al.) discloses a third method of producing a cBN sintered compact using alkaline earth metal boron nitride as a catalyst. In the third method disclosed in U.S. Pat. No. 4,772,575, a sintered compact of cubic boron nitride is made by adsorbing and/or diffusing 0.005 to 1.000 percent by weight of water into a boron nitride compact containing alkaline earth metal boron nitride as a catalyst.
The present inventor has experimentally produced a cBN sintered compact by the method disclosed in said U.S. Pat. No. 4,772,575 using the alkaline earth metal boron nitride as a catalyst, and made a cutting test with the so-formed sintered compact. As the result, it has been recognized that the so obtained cBN sintered compact was extremely worn since cBN grains fell out of the grain bond and transgranular rupture occurred, so that the expected performance was not attained. The maximum thermal conductivity of said cBN sintered compact was 6 W/cm..degree.C., which is twice or three times better than that of BeO (beryllium oxide) or AlN (aluminum nitride) generally used as a heat sink material. However, it has also been recognized that said cBN sintered compact had a rather largely dispersed thermal conductivity, which is the most important basic property of a heat sink material.
In order to solve this problem, the inventor has studied the cBN sintered compact obtained by the method disclosed in said U.S. Pat. No. 4,772,575 (Ota et al.), to recognize that unit grains forming said cBN sintered compact were irregular and this disadvantage even increased as abnormal grain growth was increased. Such irregularity of the grains easily leads to falling of the grains out of the grain structure and transgranular rupture (cleavage), to reduce the wear resistance and strength (toughness) of the sintered compact. Further, the density of the sintered compact is relatively low, whereby phonon scattering is increased at the grain boundaries and the thermal conductivity is reduced.
The method of U.S. Pat. No. 4,772,575 uses the alkaline earth metal boron nitride as a catalyst for converting hBN to cBN in a eutectic state with BN, and hence states for the generation of cBN cores and the following growth of cBN crystal grains vary significantly with the dispersed state of the catalyst, with the treatment temperature and even with small pressure variations. According to said method, therefore, an abnormal grain growth of cBN grains easily takes place at least partly and hence it is extremely difficult to control the configurations and sizes of the grains.